On what platform? I've been using Firefox and uBO on Linux and Android for over a decade and never seen a YouTube ad.

They do staged rollouts, maybe a-b testing. It seems to generally be region based rather than platform.

You might need to check that you are using all appropriate blocklists as well. The subreddit usually has a sticky/pinned post for YouTube related issues as this has been a slow moving target for about a year now.

Do you have other extensions? For example, I can see that uMatrix is also blocking all requests to doubleclick.net

I'm on Windows 10.

Ubuntu.

I've not seen the ads at least not yet.

There was a few times when Youtube try to top me from viewing because I was blocking the ads.